The research goals of the Section of Molecular Neuroscience are to define the molecular mechanisms underlying the development and function of mammalian chemosensory systems. Research efforts this past year have been directed towards establishing functional assays for bitter taste receptors and identifying and characterizing novel genes selectively expressed in taste receptor cells. To better understand the specificities and signal transduction pathways of the G-protein coupled bitter receptors, we have cloned and constructed baculoviral expression vectors with the 23 identified human bitter receptors. In situ re-constitution experiments with membranes isolated from cells infected with these baculoviruses indicate that the membranes are highly enriched in functional bitter receptors. Thus far, ligands to two orphan bitter receptors have been identified by screening responses of these receptor-enriched members to a panel of 70 bitter-tasting compounds. In addition to the continued screening for receptor-ligand interactions and the quantitative characterization of these interactions, we are using the in situ re-constitution system to examine the G protein selectivities of bitter receptors. An ongoing effort of the laboratory is to identify genes involved in taste cell signal transduction and function. We previously generated a normalized, subtracted cDNA library from taste tissue. Sequence analyses of 20000 clones from this library indicate that it is highly enriched in taste receptor cell specific genes. In situ hybridization expression studies with selected clones are continuing and have led to the identification of several genes specifically expressed in taste cells. Two of these genes, which have been the focus of study this past year, encode putative components of the taste cell signal transduction pathway including a novel ion channel and a novel G-protein coupled receptor. Extensive in situ hybridization studies and Northern analyses indicate that both of these genes are specifically expressed in a distinct subset of taste receptor cells. For each of these genes, full length cDNAs clones have been isolated, and cell-based heterologous expression systems, as well as knock-out mouse models, are being developed.